Graphene’s
two dimensions
offer new physics,
novel electronics
By Alexandra Witze
Some physicists spend their days exploring the three dimensions of space, the four dimensions of spacetime or
even the 11 dimensions of something
called M-theory. Other researchers are
content with just two.
But fewer dimensions doesn’t mean
less science. For seven years, researchers
have been enjoying a two-dimensional
playground of ne w physics provided by a
superflat material called graphene.
This deceptively simple substance
— nothing more than a sheet of honeycombed carbon atoms, which you can find
within flakes from pencil lead — contains
head-slappingly bizarre physics. Unlike
almost any other common material,
graphene sometimes behaves according
to the weird rules of quantum mechanics.
And electrons within it assume an otherworldly identity, zipping along as if they
have no mass.
OppOsite page: muller lab/Kavli institute at COrnell fOr nanOsCale sCienCe;
this page: tOp twO images: e. feliCianO; bOttOm image: 3drenderings/shutterstOCK
“Suddenly graphene came on the
scene and it had a completely new physics to it,” says Joseph Stroscio, a physicist
at the National Institute of Standards
and Technology in Gaithersburg, Md.
“That got everyone very excited” — even
scientists who ordinarily like lots of
dimensions.
Discovered in 2004, graphene was
quickly recognized as cool enough
to warrant a Nobel Prize in physics,
awarded in 2010. Now, researchers are
shifting from simply being excited about
graphene (SN: 9/27/07, p. 200) to more
deeply understanding and even harnessing the physics at play.
In this false-color microscopy image, a
patchwork “quilt” of graphene displays
colorful patches where the usual six-member carbon rings grow imperfectly
and at different orientations.
For one thing, scientists now understand how stacking one sheet of graphene
atop another in just the right way can
change the way electrons flow between
the layers. Other researchers have found
that putting graphene atop a slab of boron
nitride lets them manipulate the electron
flow far better than before. Some groups
are already designing devices for a new
graphene age; this spring, IBM researchers reported building the first electronic
circuit entirely out of graphene.
Graphene, the two-dimensional wonder material, seems ready to deliver on
some of its early promises for the three-dimensional world.
A simple sheet Not all of graphene’s predecessors have lived up to their original hype. In 1985, Texas and British chemists discovered cages of carbon atoms including the famous “buckyballs,” 60-carbon con- glomerates that look like miniature soccer balls. These molecules, part of a class called fullerenes, were touted as
the next big thing in electronics; yet after
a quarter century, nobody has a bucky-ball running an iPhone. Then, in 1991, a
Japanese scientist discovered another
carbon curiosity, tiny “nanotubes” made
of rolled-up atoms. Although some scientists have developed new electronics based on carbon nanotubes (SN:
12/4/10, p. 20), the tubes turned out to
be hard to make and arrange cleanly.
Now graphene is having its try at a
technology revolution, and many argue
it will fare better than the other carbon
protégés. “The thing about graphene is
that it’s a truly t wo-dimensional crystal,”
says Antonio Castro Neto, a theoretical
physicist at Boston University who is
setting up a new graphene research cen-
ter at the National University of Singa-
pore. “We never had something like that
before.”
In graphene, electrons can flow far
more freely than they can in either
buckyballs or nanotubes, in part because
it is the simplest of these forms of
carbon. The bonds between the atoms
also make graphene superstrong and
superflat; in theory, a 1-meter-square
Graphene
Fullerenes
Nanotubes
Carbon siblings the single layer of
honeycombed carbon atoms that make up
graphene forms the basis of two other carbon
materials of scientific interest: ball-shaped
fullerenes and curled-up nanotubes.
hammock of graphene could support the
weight of a cat despite being lighter than
the cat’s whisker.
Rather than forming as individual
sheets, graphene forms as layer after
layer within graphite, the stuff of pencil lead. One millimeter of graphite contains roughly 3 million layers of stacked
graphene. “If you write very carefully,
it’s likely you’ll get a few layers of graphene from your pencil,” says Sankar
Das Sarma, a physicist at the University
of Maryland in College Park.
In keeping with the office-supplies
theme, the scientists who won the Nobel
for graphene used Scotch tape to pull
apart flakes of graphite. By repeatedly
folding and then opening up a piece of
tape with graphite stuck on, Andre Geim
and Konstantin Novoselov of the University of Manchester in England managed to peel off single graphene layers.
Each single layer, the scientists later
found, behaves in extraordinary ways.
In most materials, the speed of electrons
changes with their energy. In graphene,
though, electrons behave as if they have
